Refrigerating systems operating at temperatures below freezing will require defrosting at one time or another. On systems operating below zero degrees Fahrenheit, one common method of defrost is known as hot gas defrost. During the hot gas defrost cycle, the liquid refrigerant (typically ammonia in larger systems) must be pumped out of the evaporator coil, and replaced with warm high pressure ammonia gas. The evaporator is essentially converted to a condenser during this cycle. Upon completion of the defrost cycle, the high pressure gas must be released from the coil back to the compressor suction line. The large volume of compressed gas and the high pressure differences between the evaporator coil and the suction line require the gas be released in a controlled manner to avoid the possibility of vapor propelled liquid in the system. Industry guidelines recommend the use of a small bleed down solenoid valve piped in parallel to the suction stop valve, or valves which open in steps. When incorporating a bleed down solenoid, the smaller valve is opened prior to the suction stop valve for a period of time to allow the pressure in the coil to bleed off gradually. This method is strictly based on time. The use of a bleed down solenoid can still result in problems if the bleed valve does not actually open, or the pressure does not bleed down to a safe level prior to opening the suction stop valve.